Gas turbine engine blades typically have dovetails or roots carried by a slot in a metal rotary disk or drum rotor. A typical blade 1 is displayed in FIG. 1, showing an airfoil section 2, and root section 3. The root section 3 provides the means by which the blade is attached to the rotor disk or other similar component of a gas turbine engine or comparable piece of turbomachinery. The blade 1 may also include an interface 4 between the airfoil 2 and root 3, to conform to the rotor disk or other attachment mechanism.
Composite blades have many advantages over blades made with other materials, such as current metal alloys. They have a high strength to weight ratio, which allows for the design of low weight parts that can withstand the extreme temperatures and loading of turbomachinery. They can also be designed with parts with design features not possible with other materials (such as extreme forward sweep of compressor blading). A major drawback of composite blades is that their strength is essentially unidirectional. Despite having a relatively high uniaxial tensile strength, the composite materials are fragile and weak under compression or shear. However, in gas turbines, the blades are usually under extremely high tensile loads. Problems usually arise with regard to the transfer of such loads into the disk. Since the blades are often made of a laminated fiber or filament reinforced composite material, and the disks are typically made of metal, the transfer of loads between the two can lead to damage of the fibers, or even worse, delamination of the composite materials.
FIGS. 2a-2c illustrate the problem, where there is shown three separate views of an example of a composite blade root. First there is an unloaded blade 10a in FIG. 2a. Then, when tensile loading T is applied as shown, there is the case of loaded blade 10b in FIG. 2b, where shear stresses have caused failure of the root structure. Or, there is the case of loaded blade 10c in FIG. 2c, where the resulting stresses from tensile load T as applied to the blade from the surrounding disk cavity (not shown) have caused delamination of the blade. The challenge therefore, is to provide an optimum load path between the composite blade structural fibers and the surrounding disk.
Previously, one of the technology barriers for high performance composite blades has been designing an attachment scheme that would utilize the strengths of composite materials to prevent the failure illustrated in FIGS. 2b-2c. As demonstrated in FIGS. 2a-2b, a critically important area is the blade attachment region or “neck” portion 11 of the blade, where the thicker root transitions out to the relatively thin airfoil section above. It is this portion which tends to delaminate or otherwise fail, when the blade is loaded and the resulting stresses are applied to the root and interface between the root and disk. One reason for such failures is that the disk lugs tend to separate due to both the centrifugal loads acting on the disk and blade. FIG. 2d illustrates another example of a blade 15 inserted into a disk 16, under no loading. The disk lugs 17 around the neck 18 of the blade 15 define a gap G0 that conforms to the shape of the blade 15. In FIG. 2e, the blade of FIG. 2d is shown under centrifugal axial loading, where the gap has increased in size to GL. Although this geometrical change in the disk geometry is slight (the dimensions portrayed in FIGS. 2d-e are exaggerated for effect), it no longer conforms to the shape of the blade. The effect of this slight increase in gap induces transverse tension and/or shear stresses.
Since composite materials have little ability to handle transverse tension or shear loading, this will result in failure of the composite blade as in blade 10c once the intralaminar tension or shear stresses exceed the ultimate intralaminar stress capabilities of the composite. An example would be uni-directional Kevlar composite, having an ultimate intralaminar stress capability of about 6 ksi.
Since composite blades are very useful in gas turbine engines, it is desirable to provide a tailored attachment mechanism of composite airfoils, that both takes advantage of the relatively high tensile strength of composites, and minimizes the disadvantage of the relatively low shear and transverse tension strength of composites.